Galvanic coating process



June 29, 1948. I TARR ETAL 2,444,174

GALVANIC COATING PROCESS Filed Aug. 24, 1943 aa vwu7mw Patented June 29, 1948 UNITED STATES PATENT OFFICE GALVANIC COATING PROCESS Allan L. Tarr, Las Vegas, Nev., and George W.

Oxley and Frank C. Fyke, Elizabeth, N. 1., assignors to Standard Oil Development Company, a corporation of Delaware Application August 24, 1943, Serial No. 499,830

of corrosion; In other words, the corrosion of common metals in solutions is electrochemical in nature rather than a simple dissolution or chemi- 09.] action. a l

One of the principal ways by which this action occurs is through the dissimilarity of two metals in electric contact in an electrolyte.

This action results from the fact-that any metal, when submerged in anelectrolyte, tends to assume a potential with respect to the electrolyte. This potential is called solution potential" and every kind of metal has a definite and consistent solution potential. If two metals with different solution potentials are .placed in contact with an electrolyte and also are connected through a metallic circuit, an electric battery cell is formed and current will fiow impelled by a voltage equal to the diiference' between the solution potentials of the two metals. This current will flow through the electrolyte from the metal with a greater solution pressure, to the metal with the lesser solution potential. This current flow is accompanied by the movement of anions through the electrolyte to the less noble metal and the movement of hydrogen ions to the more noble metal where hydrogen may be liberated from the electrolyte.

i particularly true in the case of cargo tanks in tankers during the ballast voyage. Some of the oil tanks in these tankers are filled with sea water as ballast to take the place of the oil carried on the outgoing voyage. The salts contained in the sea, water are particularly active in attacking the metal of'the tanks. Furthermore, the ferrous products of corrosion themselves hasten the corroding action, because these products set up new galvanic couples with the surfaces of the tank on which the products of corrosion accumulate.

The purpose of electrolytic protection is to provide an artificial current which overcomes the small circulating currents preventing 'or extinguishing the anodic areas and rendering most or all of the metallic areas cathodic. This can be done by providing artificial electrodes of a less noble metal whose solution potential is greater than that of the metal to be protected. v,Among the common metals whose solution potentials are greater than that of iron may be mentioned zinc,

magnesium and aluminum. Of these, zinc is preferred for the present process and apparatus as it i cheaply obtained, easily installed, and is in other respects particularly adapted to the present invention. However other metals such as magnesium, anodic to steel, might be used.

The use of zinc for the galvanic protection of ferrous objects against corrosive action of liquids is not broadly-new. For example, one method of protection consists in suspending one or more bars of cast zinc in the corroding medium while maintaining the other end of the bars in direct metal lic contact with the ferrous surface to be protected. This method of using cast zinc possesses the disadvantage that the surface of the anode becomes badly pitted and is soon covered with a hard cement-like coating formed from the products of corrosion of the zinc. This gradually reduces the efliciency of the electrode and necessitates manual cleaning which is almost impossible due to the tenacity with which the scale adheres to the surface of the zinc. Furthermore, when the method is applied to cargo tanks, the mechanical cleaning of the anodes is even more expensive and often not practical. Either the anodes must be built very light and small so that they can be readily removed from the tank, or the cargo tanks themselves must be completely gas-freed so as to permit a man to enter the tanks and clean the anodes manually. Either of these expedients is dificult and ineflicient as the cleaning must be done at very short intervals. Furthermore the electrodes may not be easily accessible and gas freeing may not be feasible.

Therefore, one object of the present invention is to overcome the foregoing difliculties and provide an electrode of simple construction and low cost which can be used for long periods without replacement and without cleaning, which will provide the maximum amount of protection per unit weight of zinc.

Thus the invention will provide an efiicient method for the galvanic protection of the interior of ballasted cargo tanks in sea-going oil tankers during the ballast voyage.

A further object of this invention is to provide a novel electrode of simple construction designed to prevent the accumulation of products of corro- -sion thereon.

The zinc electrode, according to the present invention, comprises a thin flexible sheet of wrought zinc having a thickness of 3 s" to 3 and an effective surface equal to between 1 and 3%, preferably about 2% of the exposed surface of the tank tobe protected, theelectrode thus having a large proportion of area for a given weight. This sheet may be installed as a flat plate with or without reinforcement to prevent premature failure during cleaning and gas freeing or it may be installed as a cylinder or in any other desirable manner. When an electrode of this type is suspended in a tank containinga corrosive medium such as sea water, complete protection of the exposed surfaces is obtained and there is negligible fouling or polarization of the zinc anode. Furthermore, the steel surfaces of the tank become covered with a protective coating which effectively protects the tank long after the galvanic current has ceased as would be required in an unballasted compartment. Ballasting might be staggered to assure protection of to the supports 4 by means of steel cables 5, or

equivalent means, is a thin flexible wrought zinc electrode 2. This electrode is submerged in seawater 3, maintained as ballast in the tank. A suitable electrical connector 6, which may comprise a copper wire suitably protected by an insulating coating, is used to connect the electrode 2 and the tank structure I or 9. The electrical conductor can be attached to the tank and to the electrode in any suitable manner was to secure good electrical contact.

The protection obtainable with this electrode is indicated by the following: two single sheet zinc electrodes 3" x 30 x 5 were suspended completely immersed in sea water in a model center cargo tank having about 15,000 square inches of exposed surface to be protected. The tank was filled with sea water so as to completely cover the two electrodes. After three weeks, the zinc electrode was found to havecorroded evenly over its entire surface, which was relatively smooth with no evidence of scale formation or pitting. Furthermore, the inside surface of the tank was found to be covered with a protective coat which continued to protect the tank long after the galvanic current had ceased. The effectiveness of the protection thus obtained by this novel type electrode is indicated in Table I showing the very low rates of corrosion of test plates removed from the inside walls of the tank after three weeks exposure to the action of the galvanic current from a zinc electrode fabricated Another test panel, which had been fastened to the inside of the tank and subjected to galvanic protection for about two months during which time it acquired a hard protective coat, was subsequently immersed in seawater in another tank without any galvanic protection for about 19 months, at the end of which time it showed a. corrosion loss of only 0.0013" per year. In contrast to this, the bottom of the exposure test tank itself was corroded to such an extent that it had to be replaced.

The exact composition of this protective coating is not understood, but it is known to be a complex mixture of inorganic salts of zinc and magnesium with considerable amounts of calcium.

The deposition of this coating on the steel walls of the tank results in a gradual decrease in current flow and consequently results in a corresponding decrease in the consumption of zinc from the electrode. As indicated in the foregoing the coating affords protection after the galvanic current has ceased, consequently the current density at the anode upon resumption of the protection is about the same as it was when the flow of protective current ceased. This is illustrated by the data presented in the following Table II which gives current readings obtained during two ballast voyages of a tanker in which two 32 ft. x 2 ft. x inch sheet zinc electrodes made from wrought zinc were installed in a center cargo tank of a seagoing tanker.

TABLE II Current Readings, Amperes Day Port Side Starboard Electrode sld e Electrode AAA :4 s vvv 1 Sea water pumped out. 1 Cargo of gasoline taken aboard. 3 Sea water ballast resumed.

These data clearly show that the galvanic current decreases with time indicating that the steel surfaces of the tank become polarized with a protective coating which effectively prevents corrosion of the metal.

This invention has been described in reference to the protection of tanks in seagoing steel tankers, but it should be understood that the novel electrode herein described is suitable for a great many other industrial applications; for instance, aluminum tanks used for crystallizing tartaric acid from solution can be protected by a galvanic method using the sheet electrode of the present invention. Storage tanks, boiler and refinery equipment, cast iron gas coolers, condensers and power plant equipment and, in general, any metal subjected to the corrosive action of an electrolyte can be protected in a like manner.

The nature and objects of the present invention having thus been set forth and specific illustrative embodiments of the same given, what is claimed as new and useful and desired to be se cured by Letters Patent is:

The process of forming a protective coating on ferrous metals in contact with seawater which consists of the steps of contacting the said ferrous metals to seawater, positioning in the seawater a sheet of wrought zinc having an effective surface of about 2 of the ferrous surface to be protected, and electrically connecting said zinc sheet to said ferrous surface, whereby, the galvanic action developed causes deposition on the ferrous surface of a firm dense protective coating consisting of inorganic salts of zinc, magnesium and calcium.

ALLAN L. TARR. GEORGE W. OXLEY. FRANK C. FYKE.

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